1. Field of the Invention
This invention relates to the demetallation of hydrocarbon materials, and more particularly to the removal of arsenic from shale oils.
2. Description of the Art
Vast deposits of oil shale, a sedimentary marlstone, are known to exist in various areas of the world. Such deposits are found in the United States, with the more commercially important materials located in the states of Colorado, Utah and Wyoming. The geologic unit known as the Green River formation in those states contains oil shale having up to about 35 percent by weight of hydrocarbons, in the form of kerogen. Upon heating the shale ("retorting"), kerogen decomposes to produce crude shale oil vapors, which can be condensed into a synthetic crude oil and subsequently introduced into a refinery for conversion to valuable fuels, lubricants and other products.
A number of retorting processes are known, generally classified in two categories: "in situ", wherein shale is heated in chambers formed underground without removing a significant portion of the rock material, and "above ground", wherein shale is mined by conventional methods and transported to a pyrolysis device for heating. The various processes each accomplish separation of solid and liquid retort products, using techniques which are specifically designed for the particular process.
One successful above ground retoring process is shown in U.S. Pat. No. 3,361,644 to Deering, which patent is incorporated herein by reference. In this process, oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale continuously exchanges heat with a downwardly flowing high-specific-heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 1,200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In the lower section (the preheating zone), the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier hydrocarbons condense in this lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain additional product oil. The remaining gases are then utilized as a product gas, a recycle gas as hereinbefore described, and a fuel gas to heat the recycle gas to the previously specified 1,200.degree. F. temperature.
In all known oil shale retorting processes, arsenic components of the shale either sublime to or are pyrolyzed into vaporous arsenic-containing components. As a result, arsenic in various forms collects with the educed hydrocarbonaceous vapors and condenses with the higher molecular weight hydrocarbons in the preheating zone or, in some processes, in a condenser situated outside of the retorting vessel. When oil shale from the Green River formation is retorted, the concentration of arsenic in the produced crude shale oil is usually in the range of about 30 to 100 parts per million by weight.
Shale oil can be refined to produce valuable fuels, lubricants and the like, using many of the methods known for petroleum processing, such as catalytic cracking, hydrotreating, hydrocracking, reforming and others. Problems arise, however, due to the irreversible poisoning of expensive catalysts used in such processing, caused by the high arsenic content of the oil.
In addition to causing processing difficulties, the arsenic content limits the usefulness of shale oil even in its unrefined state, since burning an arsenic-containing fuel results in unacceptable pollution. For these reasons, it is desirable to reduce the amount of arsenic present in shale oils to the lowest possible level.
Murray et al., in U.S. Pat. No. 2,779,715, describe an arsenic-removing treatment for hydrocarbons, which requires mixing the hydrocarbon with an alkali metal or alkaline earth oxide, hydroxide, or salt which will have a pH above 7 when dissolved in water. Upon separation of the hydrocarbon, it was found to have a reduced arsenic content.
U.S. Pat. No. 2,867,577 to Urban et al. teaches a method for removing arsenic from hydrocarbons by treating with a nitrogen compound, such as ammonia, hydrazine and amines, and separating a hydrocarbon with reduced arsenic content.
Other arsenic removal methods have utilized solid absorbents, such as nickel and molybdenum components deposited on refractory oxides. Examples of such methods are U.S. Pat. Nos. 3,804,750 to Myers et al., 3,876,533 to Myers, and 4,046,674 to Young.
Young, in U.S. Pat. No. 4,075,085, describes a method wherein a hydrocarbon feedstock is mixed with oil-soluble nickel, cobalt or copper-containing additives, heated to at least 300.degree. F., and filtered to remove arsenic. This method has been applied to crude shale oils.
A need remains for a simple, inexpensive method for reducing the arsenic content of shale oils. Such a method would preferably utilize materials which are readily available at locations where crude shale oils are refined.
Accordingly, it is an object of the present invention to provide a method for removing arsenic from shale oil materials.
Another object is to provide an arsenic removal method for shale oil which consumes by-product materials from the refining of the oil.
These and other objects will appear to those skilled in the art, from consideration of the following description and claims.